Carbon fiber reinforced composite materials are useful since they are excellent in strength, stiffness, conductivity, etc., and widely used for an aircraft structural member, a windmill wing, an automotive outer panel and computer uses such as an IC tray or a housing of notebook computer and their needs are increasing year by year.
The carbon fiber reinforced composite material is generally an inhomogeneous material obtained by molding a prepreg of which essential constituting elements are a carbon fiber which is a reinforcing fiber and a matrix resin, and accordingly, there is a big difference between physical properties of arranging direction of the reinforcing fiber and physical properties of other direction. For example, it is known that an impact resistance expressed by a resistance to drop impact is, since it is determined by delamination strength which is quantitatively measured as interlayer edge peel strength, not resulted in a drastic improvement only by increasing strength of the reinforcing fiber. In particular, carbon fiber reinforced composite materials of which matrix resin is a thermosetting resin has, in reflection of a low toughness of the matrix resin, a property to be broken easily by a stress from other than the arranging direction of the reinforcing fiber. Accordingly, various means are proposed for the purpose of improving physical properties of composite material capable of resisting to the stress from other than the arranging direction of the reinforcing fiber.
As one of them, a prepreg provided with a resin layer, in which resin particles are dispersed, on surface region of the prepreg is proposed. For example, a method for providing a high toughness composite material excellent in heat resistance, by using a prepreg provided with a resin layer in which particles consisting of a thermoplastic resin such as nylon are dispersed in surface region of the prepreg, is proposed (refer to Patent reference 1). And, other than that, a method for developing a high toughness of composite material by a combination of a matrix resin of which toughness is improved by adding a polysulfone oligomer and a particle consisting of a thermosetting resin is proposed (refer to Patent reference 2). However, these methods give a high impact resistance to carbon fiber reinforced composite material on one hand, but on the other hand, result in producing a resin layer to become an insulating layer in the interlayer. Accordingly, there is a defect that the conductivity in thickness direction, among conductivities which are one of characteristics of the carbon fiber reinforced composite material, significantly decreases, and it was difficult to make an excellent impact resistance and conductivity compatible in the carbon fiber reinforced composite material.
Furthermore, as methods for improving conductivity of the interlayer, a method of compounding a metal particle to a matrix resin of carbon fiber reinforced composite material (refer to Patent reference 3), or a method of compounding a carbon particle (refer to Patent reference 4) can be considered, but in these references, no reference is made to a compatibility of an excellent impact resistance and conductivity.    [Patent reference 1] specification of U.S. Pat. No. 5,028,478    [Patent reference 2] JP-H3-26750A    [Patent reference 3] JP-H6-344519A    [Patent reference 4] JP-H8-34864A